The peptidergic innervation of arterial chemoreceptor organs (the rat carotid body and vagal paraganglia; guinea pig carotid body) was studied immunohistochemically. Five different populations of nerve fibres in the guinea pig carotid body could be discriminated according to their origin and their chemical coding. The innervation pattern of the rat carotid body differed in some aspects. Comparison of the rat carotid body and vagal paraganglia suggested that autonomic neuropeptide Y-like immunoreactive fibres act primarily via vascular mechanisms rather than directly on the chemoreceptor tissue. Sensory fibres were shown to contain immunoreactivities for substance P, calcitonin gene-related peptide (rat and guinea pig) and somatostatin (guinea pig). The functional role of the identified peptide-containing sensory fibres remains to be established.